The invention relates to a circuit arrangement for increasing the definition of color contours during the display of a color television signal formed from a luminance signal and at least one color difference signal, comprising a slope detector which detects the slope of the luminance signals and produces a detection signal when the slope exceeds a predetermined value, a control circuit controlling at least one color difference signal edge enhancement circuit formed by a change-over switch and a store, and a delay circuit for the luminance signal for providing coincidence with the edge enhanced color difference signal.
The picture resolution in contemporary television transmission systems is limited by the prescribed transmission bandwidth. The color difference signals are transmitted with a bandwidth which amounts to only approximately one fifth of the bandwidth of the luminance signal. The color resolution in the television picture is correspondingly low. This is more specifically noticeable at contours. The color transitions are even approximately five times longer than the associated luminance transitions corresponding to the smaller transmission bandwidth of the color difference signals.
In order to improve the resolution of the contours, it is therefore necessary to increase the comparatively flat color signal slope at the edges. Such a circuit arrangement of the type defined in the opening paragraph to increase the resolution at color contours is disclosed in German Auslegeschrift No. 1562170. In the circuit disclosed in said Auslegeschrift, the luminance signal is applied to a slope detector which produces a detection signal when the slope in the luminance signal is steep enough. The two color difference signals are applied to the change-over switch via a delay circuit which has a time delay approximately equal to half the time of the maximum slope (half the rise time) of a color difference signal. The output of the slope detector is connected to a first pulse shaper which, at the occurrence of the detection signal, briefly closes a first switch, so that the output of the delay circuit is connected to the input of the store. Also, a control circuit is energized, which switches the change-over switch such that the output of the store constitutes the output of the change-over switch. After half a rise time, a second pulse shaper briefly closes a second switch which connects the input of the delay circuit to the input of the store. After a further half rise time, the control switch resets the change-over switch.
Since the luminance signal has a significantly larger bandwidth than a color difference signal, it may happen that during a long rise time of a color difference signal a plurality of sudden transients occur in the luminance signal. Although an edge enhancement of a color difference signal is unwanted then, the slope detector supplies a detection signal for producing a color difference signal with a steeper slope. In addition, a switch of the control circuit arranged between the first pulse shaper and the slope detector is opened when a detection signal has occured, and is closed again after the edge enhanced color difference signal has been generated. This circuit configuration produces color purity errors in the television picture.
German Offenlegungsschrift No. 32 23 580 also discloses a circuit arrangement for increasing the resolution of color contours, in which a color difference signal is entered into a store via a switch and is also applied to a slope detector which produces a detection signal when the slope exceeds a predefined value. At the occurrence of the detection signal the switch is opened, and after the rise time of the color difference signal has ended, the switch is closed again. The store consequently supplies the edge enhanced color difference signal. The attendant delay in the color difference signal is compensated for by a corresponding delay in the luminance signal.
In the commonly used video recorder systems the bands of the luminance and color difference signals are additionally limited during recording. This causes the rise time of the color difference signal to be prolonged on display. In practice it has been found that a circuit arrangement such as it is described in DE-OS No. 32 23 580 does not always detect the vertical color contours in the same position in a line, so that color contour distortions occur in the picture.